High intensity light sources, and particularly white high intensity light sources, are interesting for various applications including spots, headlamps, stage-lighting and digital light projection. For such purposes, it is possible to make use of so-called light concentrators where shorter wavelength light is converted to longer wavelengths in a highly transparent luminescent material. Such a transparent luminescent material can be used and is illuminated by LEDs (Light Emitting Diodes) to produce longer wavelengths within the luminescent material. Converted light, which will be waveguided in the luminescent material, is extracted from a surface leading to an intensity gain.
In digital projection it is desirable to dim the screen brightness, for example for saving energy and preserving lifetime of the light source. In the solution described above, a bar of the luminescent material is placed on top of a row of LEDs. Due to losses such as light leakage as a result of scattering and re-absorption as the LED (pump source) is further away from the exit surface its contribution to the total light output becomes less.
Consequently, the dimming is inefficient and inadequate for many practical purposes, particularly within digital projection but also in the area of e.g. spot lights, headlamps and stage-lighting.
US 2012/0243220 A1 describes a light emitting apparatus comprising a plurality of light emission groups each comprising a plurality of LEDs. Each light emission group is associated with a transparent encapsulation and a wavelength converting member. The light emitting apparatus further comprises drivers for collectively controlling the switching on and off and the dimming operations of the LEDs belonging to each light emission group.
In the above prior art solution, however, only relatively few degrees of freedom of dimming, i.e. levels of intensity of emitted light below maximum, are available and the different degrees of dimming are separated by an inconveniently large difference in light intensity.
US2012/0212931A1 discloses a light emitting device includes a first light source, an optical waveguide body, a light emitting layer and a first reflection layer. The optical waveguide body includes a first end surface to which light from the first light source is injected, and a second end surface opposed to the first end surface and provided in a light guiding direction of the light. The light emitting layer includes, along the light guiding direction, phosphor particles capable of absorbing the light and emitting wavelength converted light or a light diffusing agent diffusing the light. The first reflection layer is provided on the second end surface and is capable of reflecting part of the light guided in the optical waveguide body. Diffused light from the light emitting layer is emitted to outside of the optical waveguide body. A controller is provided that can lighten light sources in an order such that a light emitting region moves.
US2011/0175549A1 discloses a linear light source, which emits light by causing light to propagate in a linear material in which incoming light can be propagated. A light emitting device is provided on a side of one end of the linear material which emits light so that the light enters the one end of the linear material and a light receiving device is provided on a side of the other end of the linear material, which detects the light which has propagated in the linear material.